As set forth in the above referenced disclosure, incorporated by reference, a laser device can be used to encode strain or other measured variations occurring within a structural member in the form of a laser beam. More particularly, a structural member which is typically loaded with compressive or tensile stress will routinely flex or yield in response to the stress loading. If in tension, the member will elongate, thereby forming strain which is proportional to or related to the stress placed on the member. Stress in the opposite direction causes strain of the opposite sign. The present disclosure sets forth particular and specific embodiments of strain encoding lasers which thereby emit such laser beams. That is, the present apparatus describes specific arrangements of laser devices converting the strain or other measured variations of the monitored structural member into a laser beam frequency variation which can be easily transmitted away from the member, thereafter intercepted at a remote location, decoded and interpreted. In a manner of speaking, it provides a type of strain (or measurement) gauge which is attached to the structural member undergoing tests wherein the signal from the gauge is coupled away from the structural member free of entangling wires or cables. Therefore the disclosed laser device can be used as a measurement system, or as a telemetry system, or both as a measurement and telemetry system.
The present apparatus sets forth a laser body which defines a solid body of active material capable of lasing action wherein the body is cooperative with two or more reflective surfaces, or mirrors, thereby encoding the measured strain inforamtion in an emitted beam from the laser. The reflectivity of the mirrors is adjusted to assure that the beam is emitted from a selected end face or faces of the body of lasing material. Moreover, a separate or remote (external) reflective mirror can also be included and is installed parallel to the faces of the laser body. This external reflective mirror is incorporated to direct the laser beam from a remote point back into the body for modification of the coherent beam generation occurring in the body. There is a space or gap established between the laser body and external mirror, the space or gap enabling the mirror to be affixed remote from the lasing body. This spacing for mirror and laser body enables the geometry of the measuring device to be modified for attachment to the structural member of interest, which thereby allows length L over whioh strain measurements are made to be adjusted, and allows encoding of strain in the laser frequency.
The laser beam is thus generated within the cavity comprised of lasing material, is directed out one end face thereof, is transmitted through a distance (or space) to be reflected from the external mirror, and is directed back into the laser body. This reinforces the beam generation occurring in the laser body which thereby defines coherent beam formation and beam frequency which is emitted out of the laser body. The emitted beam is then directed either from the second face of the body, or from the external mirror, encoding changes in the component spacing L so that beam frequency is a function of strain occurring in the structural member in the length L.
Recall that the external mirror and laser body are attached to the structural member. The spacing between the two components establishes a standing wave between the components. If the spacing is varied, the standing wave is varied so that a change of frequency occurs wherein frequency change is dependent on change in spacing or dL.
The present apparatus sets forth alternate embodiments including an extended cavity arrangement between the laser body and external mirror and an alternate coupled cavity arrangement. These various embodiments relate in part to the types of reflective or transparent finishes applied to the end faces of the laser body and to the external mirrors. Recall that a laser body typically operates with spaced faces wherein the coherent beam is formed between the faces.
The present apparatus is summarized as a lasing bodY equipped with parallel partially reflective end faces and a spaced reflective mirror, that could be 100% or partially reflective. The two components are supported by a mounting means on a structural member to measure stress in that member where the stress is converted into strain and the strain varies the spacing between components. When spacing is varied, the frequency is varied by spacing and hence by stress.